Tesla Semi

[mongo]

Aft or fore doesn't matter, it's the total for the axle group that matters, which is 34K per tandem axle group.

It matters for weight distribution, in this case how much of the pack weight ends up on the front axle. The pack is slightly forward of the halfway point between the forward and fore axle, but 2/3 of the way toward the forward axle compared to the aft rear axle. By using the front and aft axles as the support points, more of the pack weight is applied to the front axle. The fore rear axle is basically directly under the 5th wheel, so it can take up trailer load without unloading the front axle.

Edit: forgot an 's'

 

[dhanson865]

Thanks, that got better as it went along and liked the discussion at the end. Watch at x1.5 or even x2.0 speed tho (gear icon in youtube)

x1.5 was too fast, I found x1.25 to be perfectly normal sounding and less boring than the 1.0 speed. Still, glad you reminded me, I might have wasted more time if you didn't mention it.

 

[LCR1]

It matters for weight distribution, in this case how much of the pack weight ends up on the front axle. The pack is slightly forward of the halfway point between the forward and fore axle, but 2/3 of the way toward the forward axle compared to the aft rear axle. By using the front and aft axles as the support points, more of the pack weight is applied to the front axle. The fore rear axle is basically directly under the 5th wheel, so it can take up trailer load without unloading the front axle.

Edit: forgot an 's'

Still irrelevant. If you have 15K on the rear of the tandem and 20K on the front of the tandem that's 35K for the tandem and you are overweight regardless of which of the two axles are actually supporting the weight.

 

[jhm]

That's a great find. Very honest. I think this mechanic has a very good sense of awareness of the challenges ahead. I wish him the very best in finding new paths. Maybe he could get hired by Tesla and get out ahead on these changes.

 

[mongo]

Still irrelevant. If you have 15K on the rear of the tandem and 20K on the front of the tandem that's 35K for the tandem and you are overweight regardless of which of the two axles are actually supporting the weight.

Right, but the discussion is about getting battery weight onto the front axle, isn't it?

One of the problems they'll need to address is how to get all of the weight back on the front axle without the engine there.

So the relative position of the battery to the axles it is supported by matters. If the battery is directly over the front axle, it takes all the weight. If it directly over the aft axle, it takes all the weight. At any other point, the weight on the front axle is dependent on positioning and the individual rear axle suspension loading.
If you look at my linked write up, the front axle comes in just under the limit with a standard trailer.

 

[jerry33]

If you look at my linked write up, the front axle comes in just under the limit with a standard trailer.

If it didn't there would be some big problems.

 

[LCR1]

I went back and found your MS paint post. You mention that the 5th wheel is over the fore axle, that's irrelevant, hitches move. You mention loading the fore axle up to 20Klbs, that’s irrelevant also as again the hitch moves and being in a tandem a setup you need to calculate based on the tandem weight you will never have exactly all of the weight on one axle of the group. You also don’t want the steer axle to be maxed out because that leave no room for loading, what happens when you have a load that has a lot of pin weight? You’d can’t slide the plate back because you’d be over on your drive axles, if you moved forward you’d be over on your steer axles with only 300# of room. You’d also have to account for various driver weights and all of the stuff the keep in the cab. Also, the battery attachment point isn't going to be onto an axle, the axles move and that isn't good for the pack. They'll be attached to the frame at some point. And most likely inbetween the axles and frame.

 

[mongo]

I went back and found your MS paint post. You mention that the 5th wheel is over the fore axle, that's irrelevant, hitches move. You mention loading the fore axle up to 20Klbs, that’s irrelevant also as again the hitch moves and being in a tandem a setup you need to calculate based on the tandem weight you will never have exactly all of the weight on one axle of the group. You also don’t want the steer axle to be maxed out because that leave no room for loading, what happens when you have a load that has a lot of pin weight? You’d can’t slide the plate back because you’d be over on your drive axles, if you moved forward you’d be over on your steer axles with only 300# of room. You’d also have to account for various driver weights and all of the stuff the keep in the cab. Also, the battery attachment point isn't going to be onto an axle, the axles move and that isn't good for the pack. They'll be attached to the frame at some point. And most likely inbetween the axles and frame.

I think we have a misunderstanding here. I ran the numbers to see if a tractor with 8,000 battery pack and dimensions taken from the Tesla rendering was able to use a max trailer from a standard tractor. The tractor weight sheet I found had ~11k front and 9k rear before trailer, so I simulated a trailer with a 25k 5th wheel load. With no trailer, my simulated tractor sits at about 9k front 10k tandem. If you have some other set up, I can run those numbers.

To some of your other points.
A single axle cannot be over 20k, a tandem cannot be over 34k, Both numbers matter.
The battery is fixed, I was illustrating that its location matters for weight distribution.
If you want less weight on the steering axle, you put more weight on the fore axle, that reduces trailer load on the steering and aft axles. In my example taking the fore up to 20k would put steering at just under 11k and the aft at just over 13k, 33k for the tandem.

 

[LCR1]

You're right a single axle can not be over 20K but when measuring in a tandem setup you are measuring the group weight. There is no way you're going to have a 6,000lb discrepancy within an axle group. When you look at scales you'll notice the scales are broken down into 3 sections of steer, drive, trailer. The only time they are measuring individual axles is within a spread axle group which is 40K pounds with a 10' spread, reduced 1,000lbs for every foot. So a 10' spread axle is 40K a 9' spread axle is 39K. and it's possible to be a few hundred pounds different with a 10ft spread. If you manage to get a 6,000lb difference with a tandem spread you've broken something, there's a reason it's a all measured as a tandem group.

 

[mongo]

You're right a single axle can not be over 20K but when measuring in a tandem setup you are measuring the group weight. There is no way you're going to have a 6,000lb discrepancy within an axle group. When you look at scales you'll notice the scales are broken down into 3 sections of steer, drive, trailer. The only time they are measuring individual axles is within a spread axle group which is 40K pounds with a 10' spread, reduced 1,000lbs for every foot. So a 10' spread axle is 40K a 9' spread axle is 39K. and it's possible to be a few hundred pounds different with a 10ft spread. If you manage to get a 6,000lb difference with a tandem spread you've broken something, there's a reason it's a all measured as a tandem group.

Cool, this is where we split paths. The following is my interputation of the rendered video on Tesla's Twitter:
The Tesla rear axles are fully independant. Instead of having a center equalizer, each axle end is connected to an arm with pivot at the front and airbag plus shock at the rear. Due to this, they can set the airbag pressure to load each axle to whatever the load calls for (separate pressures for each axle). So they can adjust steering axle loading via the rear axle differential loading.

 

[LCR1]

The 5th wheel isn't connected to a single axle, it's connected to the frame which weighs down on all of the axles. It doesn't matter what's connected to what, or what axles are independent, what matters is the axle loading. All trucks have air bag suspension today, they even have air cabs and air seats.

When you pull up to a weigh station with tandem axles you'll be weighed for tandem axles at 34K, it's that simple.

 

[mongo]

The 5th wheel isn't connected to a single axle, it's connected to the frame which weighs down on all of the axles. It doesn't matter what's connected to what, or what axles are independent, what matters is the axle loading. All trucks have air bag suspension today, they even have air cabs and air seats.

When you pull up to a weigh station with tandem axles you'll be weighed for tandem axles at 34K, it's that simple.

Right, 5th wheel connects to frame, frame connects to axles via swing arm and air bags.

With independent axles and separate pressure controls, you can set the axles at 17/17, 20/14 or 14/20, the tandem load is the same, but the steering axle loading is different.
If the 5th wheel is directly above one axle, having that axle pick up more of the load makes the trailer appear lighter to the other two axles. If you add load to the aft axle instead, it causes more weight on the steering axle because the 5th wheel is between them and the fore axle is carrying less.

 

[LCR1]

Doesn't really work that way. Airbags level loads and soften the ride but they do not transfer weight, there is no fulcrum on the axle/air bag to transfer weight, this can be seen at the scales.

If you air up one bag to 60psi and 'transfer the weight' you'll notice there isn't a rise in the the other bags. If the weight had transferred you should see a rise in the other bag because there is now more weight compressing the other bag and raising the bag pressure. This is the same thing on a pickup with two axles, I air up the rear bags from 10 to 100psi I raise the rear 6 inches but the front stays the same, if I had transferred weight to the front the front axle/ suspension would have sunk down from added weight, the weight would show up on the scale; it doesn't. Raising a load will will change the load triangle but there is no fulcrum in which to transfer the weight somewhere else. Making the axles in a tandem "independant" of each other doesn't make a difference, see the independence of a pickup trucks axles. The scales don't lie.

 

[LCR1]

And none of this changes the fact that a tandem axle is and always will be measured as an axle group.

 

[ggr]

And none of this changes the fact that a tandem axle is and always will be measured as an axle group.

Mongo's point is that with the load being above the middle axle, raising/lowering the rear axle changes the load on the steering axle. As a matter of physics, he's right.

 

[AZ Desert Driver]

I'm just a civilian observer - I thought that the 5th wheel disk was a hard mounted platform capable of hauling all that load, both vertical and horizontal. Did you guys just say you can slide this disk forward and backward some distance? Its not fixed over the rear axles?

 

[MP3Mike]

I'm just a civilian observer - I thought that the 5th wheel disk was a hard mounted platform capable of hauling all that load, both vertical and horizontal. Did you guys just say you can slide this disk forward and backward some distance? Its not fixed over the rear axles?

Correct. Just like the rear wheels on some (most?) trailers can slide forward/backwards depending on the load distribution to help with the weight distribution.

 

[AZ Desert Driver]

No? not only is the 5th wheel moveable (I thought it was hard welded) now you tell me that even the axles move under a trailer? I thought axles were mounted to springs and hard welded to trailer frame. I've looked under my ICE car for years and saw axle/spring/frame---so surely every other vehicle is similar. Is nothing fixed in this truckers world?

I cant make use of any of the information, but my assumptions of what was moving beside me has changed tremendously through this forum. Thanks truckers for opening my eyes a little.

 

[LCR1]

Correct 5th wheel hitches can be unlocked, moved fore or aft, then locked back in place. That's why I don't see the point of getting hung up on where it was in relation during the reveal. Trailers can also do the same if so equipped. If you look along the frame rails and see a bunch of holes that means it's a slider

 

[LCR1]

Mongo's point is that with the load being above the middle axle, raising/lowering the rear axle changes the load on the steering axle. As a matter of physics, he's right.

So in effect locking the middle axle in place like a fulcrum with the hitch being exactly centered over it then adding or subtracting air from the rear axle to adjust it? Yes that could be done to move weight but nothing significant, if at all without serious issues.

You want weight distributed between the two axles so they have consistent wear, tear and handling. This is why you'll see hitches positioned between the axles in most cases.

If you were to use axle 2 like a fulcrum something has to change with the angle meaning the nose of the trailer would either sit higher or lower than the rear. You'd also be changing the clearence of the tractor and trailer relationship as you rotate around a fulcrum. So uneven loading, off level, and clearence discrepancy between cab/tractor and trailer.